The present invention is directed, in general, to optoelectronic devices and, more specifically, to methods of testing and manufacturing micro-electrical mechanical mirrors using an interferometer.
Electrostatically actuated micro-electrical mechanical system (MEMS) devices have recently gained wide acceptance in a variety of optical communication applications. One such use is in optical switching and steering devices. In these devices, movable micro-machined mirrors are used as switching elements to direct input optical signals to desired outputs.
Looking briefly at FIG. 1, illustrated is a plan view of a mirror device 100 of a mirror array found in a conventional MEMS optical switch. As illustrated, the device 100 is comprised of a mirror 110 coupled to a gimbal 120 on a polysilicon frame 130. Typically, the illustrated components are fabricated on a substrate (not shown) by micro-machining processes such as multilayer deposition and selective etching. The mirror 110 in FIG. 1 is double-gimbal cantilevered and attached to the polysilicon frame 130. Specifically, the mirror 110 is attached to the gimbal 120 via first springs 140, thus defining a mirror axis about which the mirror 110 may be rotated. Also, the gimbal 120 is attached to the polysilicon frame 130 by second springs 150, which further define a gimbal axis about which the mirror 110 and gimbal 120 may be rotated. Through a combination of the mirror and gimbal axes, the mirror 110 may be tilted to a desired orientation for optical signal routing via application of a voltage to an electrode or similar actuation.
Typically, to accomplish such rotation, electrodes (not illustrated) are positioned under both the mirror 110 and gimbal 120. The electrodes are configured to rotate the mirror 110 or gimbal 120 in either direction about either""s respective axis. The mirror 110 or gimbal 120 rotates under the electrostatic force between the electrode and the mirror 110 or gimbal 120, and is balanced in equilibrium by the restoring force of the first and second springs 140, 150. The degree of rotation depends upon the amount of voltage applied to the electrodes. Traditionally, a degree of rotation up to about 9 degrees in either direction about either axis is achievable using a voltage of about 150 volts.
The mirrors of an optical switch are rotated to varying deflection angles by varying the amount of voltage applied to the electrodes. Conventionally, the deflection angle increases in direct proportion to the voltage being applied. An incoming optical signal reflected from the mirror 110 to an outgoing optic fiber, depending on the chosen deflection angle. As a result, if one or more mirrors within a mirror array do not achieve the deflection angle intended by a specific applied voltage, data carried within the beam may be misdirected or even completely lost.
To help alleviate the potential for such misdirected data, telecommunications manufacturers verify, or xe2x80x9ccharacterize,xe2x80x9d the individual mirrors, for various applied voltages, within the optical switch before the switch is put into operation. In addition, whether the deflection angles fall within allowable tolerances may also be determined. Perhaps the most common technique used to characterize these mirrors employs the use of an infrared camera. In this technique, a beam of light is reflected off of one mirror at a time, and at one particular deflection angle at a time. The beam is reflected onto a target where the infrared camera locates the position of the reflected beam, thus indicating the current deflection angle of that mirror. Using this process, a single characterization step for a single mirror at a single applied voltage takes about 1 second, or perhaps a little faster, to perform. The process is then repeated for the same mirror, but with the deflection angle incremented by a variation in voltage. Again the camera locates the reflected beam to characterize the new deflection angle of the mirror. This process is repeated for each desired increment in deflection angle, along both rotational axes of the mirror, before the process moves on to another mirror within the optical switch. Alternatively, all the mirrors in the array may be characterized, one at a time, at the same deflection angle before that angle is collectively incremented for the entire array and the characterization process repeated.
Due to the demand for increased switching speed and switching capacity, the number of mirrors within an optical switch is growing at an astonishing rate. Where conventional switches incorporate about one thousand mirrors, manufacturers estimate future optical switches may have mirror arrays with over four thousand mirrors. If one second is required for characterization of a single mirror at each deflection angle, and if each mirror is moveable about 9 degrees in either direction and about both rotation axes, a conventional array having about 1000 mirrors may take an excessive amount of time to characterize the entire array. Using this conventional technique, future arrays having about four thousand mirrors would likely require almost seven weeks to characterize. Even if other conventional techniques significantly improve the characterization time for each individual mirror, for instance, to one-half second each, it is still little consolation to know that characterization of the area will now only take about four weeks, rather than seven.
Accordingly, what is needed in the art is a system and related method for quickly and accurately characterizing micro-electrical mechanical mirrors that does not suffer from the deficiencies found in the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a method of testing micro-electrical mechanical mirrors. In one embodiment, the method includes simultaneously applying a voltage to each of a plurality of micro-electrical mechanical mirrors to tilt each of the plurality to a deflection angle, and simultaneously deflecting a beam from each of the plurality using an interferometer to simultaneously determine an accuracy of the deflection angle of each of the plurality.
In addition, a related method of manufacturing micro-electrical mechanical mirrors is disclosed.